**Wind energy and wind energy density**

Wind is flowing air, and moving air has kinetic energy. This kinetic energy is called wind energy. According to the above discussion on the formation of wind, solar radiation is the most important source of wind energy. Studies have shown that the total solar radiation power received by the earth is 1.7X10^{14}kW, of which 1%~2% of the energy reaches the surface of the earth and is converted into wind energy. Wind energy on the earth has the advantages of huge reserves, wide distribution, clean and pollution-free, etc., of course, it also has its limitations, such as low energy content, instability, and obvious regional differences.

The amount of wind energy in a place or the potential of wind energy in a place is usually measured by the average wind energy density. Wind energy density is the power of wind energy that the airflow passes through a unit area vertically. The wind speed changes. According to the principles of physics, for a given area S (unit m, perpendicular to the wind direction) through which the wind flows, the formula for calculating wind energy per unit time is

Where: E is wind energy (W); p is air density (kg/m3), under normal temperature and standard atmospheric pressure, it can be taken as 1.225kg/m³; v is the wind speed per unit time (m/s). Wind energy density refers to the energy contained in the wind passing through a unit cross-sectional area per unit time, also known as wind power density (P), and its calculation formula is

Wind power density P is proportional to the third power of average wind speed, so wind speed is the most important factor in wind energy calculations.

To measure the amount of wind energy in a place, it depends on the average annual wind energy. The average wind energy density E can usually be calculated by direct calculation method and probability calculation method. The direct calculation method is to divide the wind speed data measured 24 hours a day in a certain place for the whole year (or set time period) into various levels of wind speed v_{1}, v_{2},…, v_{i}, according to a certain interval (such as 1m/s), calculate the wind energy density corresponding to each level of wind speed according to formula (1-2), and then multiply it by the accumulated hours of each occurrence in the year, n_{1}, n_{2},…, n_{i}, and finally, by adding up the results of each product and dividing by the total number of hours n in the year (or set time period), the average wind energy density of a certain place in one year can be obtained, namely

The probability calculation method is to fit the distribution of wind speed frequency through a certain probability distribution function, and calculate the average wind energy density according to the integral formula. Wind speed frequency refers to the distribution of wind speed in a certain area. The Weibull formula is generally used. The probability distribution function of wind speed v is

In the formula: K is the shape parameter; C is the scale parameter. If the wind speed v is in the area where the upper and lower limits are a and b respectively, and f(v) is a continuous function of v, then the integral form of wind energy density calculation formula is

The density of air is another important factor that affects the size of wind energy. In general, the calculation of wind energy or wind energy density is based on the density of air under standard atmospheric pressure, that is, p=1.225kg/m³. In areas with high altitudes (above 500m above sea level) and areas with complex terrain, the influence of air density must be considered.

**Distribution of Wind Energy Resources in China**

China has a vast territory, and not only is the total reserves of wind energy abundant, but it is also widely distributed. According to relevant research estimates, the national average wind energy density is about 100W/m², the country’s total wind energy reserves are about 4.8×10^{9}MW, and the 10m height of onshore and offshore areas that can be developed and utilized is about 1.0×10^{6}MW of wind energy resources. Among them, there are about 2.53×10^{5}MW of onshore wind resources that have good development and utilization value.

The utilization of wind energy resources depends on the density of wind energy and the annual cumulative hours of available wind energy. According to the effective wind energy density and the cumulative hours of wind speeds of 3-20m/s throughout the year, the distribution of wind energy resources in China can be divided into four types of areas, namely, rich areas, richer areas, available areas and poor areas. Wind energy-rich areas, richer areas, and available areas all have good conditions for the use of phoenix energy, accounting for about 2 of the total area of the country. Among them, the four provinces of Inner Mongolia, Xinjiang, Heilongjiang and Gansu have the most abundant wind resources.